JP4712970B2 - Transition metal based ceramic materials and electrodes fabricated therefrom - Google Patents

Description

[0001]
Related applications
TECHNICAL FIELD OF THE INVENTION The present invention relates generally to synthetic materials. More particularly, the present invention relates to ceramic materials, in particular ceramic materials incorporating group I elements, in particular lithium, and composed of transition metal nitrides doped with metals and / or metal oxides. The invention further relates to electrodes incorporating these materials, in particular to rechargeable lithium battery cathodes.
[0002]
Background of the invention Rechargeable batteries are constantly gaining importance as a power source for various products. One important rechargeable battery type includes rechargeable lithium batteries, and as used herein, the term includes all rechargeable lithium batteries and lithium ion batteries. It is understood.
[0003]
The cathode is an important component of a lithium battery. During battery discharge and charge, lithium ions enter and are removed from the bulk of the cathode material, respectively. The performance of the battery with respect to capacity, charge rate, discharge rate and lifetime depends, inter alia, on the material properties of the cathode material. An important parameter of some cathode materials is their electrical conductivity and lithium transport properties. Stability is as important as processing cost and ease of processing.
[0004]
Prior art cathode materials for lithium batteries are mainly based on metal oxides, in particular nickel, manganese and cobalt oxides. Typical cathode materials widely used in the prior art are LiMn ₂ O ₄ , LiCoO ₂ and LiNiO ₂ . In some examples, metal nitride is used for the electrode. U.S. Pat.Nos. 5,589,298 and 5,702,843 disclose the use of certain lithium metal nitrides as cathode materials for lithium batteries, and U.S. Pat.No. 5,754,394 is a metal nitride. A configured capacitor electrode is disclosed.
[0005]
As described in detail below, the present invention relates to a unique type of ceramic material. Although their base material is generally a transition metal nitride, oxygen can be present in the material in some proportion, and in certain embodiments of the invention, oxide or oxynitride It is understood that (oxynitride) materials are important dopants in the ceramics of the present invention. The material of the present invention has a very good electrical conductivity. Lithium is highly diffused in the materials of the present invention, which can make lithium intrusion into the material easy and reversible. This material has a very stable lattice structure and this structure is retained during repeated charge and discharge cycles. As described further below, the materials of the present invention contain dopant species that greatly enhance their beneficial properties. These and other advantages of the invention will be apparent from the discussion, description and examples that follow.
[0006]
SUMMARY <br/> herein the invention, the general formula _{Li α M 1- β T β N} x O δ ( wherein, M is a transition metal; T is a dopant metal, which, as free metal Or present as one or more oxidized or nitrided compounds; x is 1 or less; δ is 0 or 4 or less; α is 3-x or less; and β is less than 0.2 The ceramic materials based on transition metals have been revealed. In certain embodiments, M is vanadium. The dopant metal can include one or more transition metals, with Group III and Group IV transition metals being particularly preferred. In some specific embodiments, the dopant comprises scandium, yttrium, lanthanum, zirconium, titanium and / or hafnium. This dopant metal can be present in the material as oxides of the aforementioned metals, their nitrides and combinations thereof.
[0007]
In one particular embodiment, the ceramic transition metal is vanadium and the dopant is based on zirconium, most particularly a mixture of zirconium and zirconium oxide.
[0008]
Furthermore, a rechargeable lithium battery is also disclosed which incorporates the aforementioned materials into its cathode.
[0009]
DETAILED DESCRIPTION <br/> present invention relates to a ceramic material having the general formula _{Li α M 1- β T β N} x O δ, where, M is preferably a transition metal host metal, In some preferred embodiments, vanadium. T is a dopant metal; β is less than 1, most preferably less than 0.2; x is greater than 0 and less than or equal to 1; δ is less than or equal to or less than 4; α is 3-x or less. Formulations of the present invention include non-stoichiometric compositions in addition to stoichiometric compositions, and all subscript values can be increased in common multiples; that is, the ratio between them It should be understood that if is maintained, it can be scaled up.
[0010]
The dopant metal T is most preferably one or more transition metals, most particularly preferably Group III and Group IV transition metals. It should be noted that the dopant metal can be present as a free metal and / or as a compound such as a metal oxide, oxynitride or nitride. For purposes of this disclosure, “dopant metals” should be those metals represented by T in the general formula. “Dopant” in the more general sense should mean those various oxygen and nitrogen compounds in addition to such metals. Certain preferred transition metal groups used in dopants are scandium, yttrium, lanthanum, hafnium, titanium, and zirconium, and further include oxides thereof. Although the addition of a relatively small amount of dopant has been found to greatly enhance the performance of the ceramic material of the present invention and does not go beyond reasoning, the dopant distorts and expands the crystal lattice of the ceramic material, thereby It is estimated to decrease the activation energy of lithium diffusion. Further, it is believed that the dopant can act as either a p-type or n-type material, thereby increasing the electrical conductivity of the ceramic material. In certain instances, the dopant can act to form a two-phase mixture that allows for faster lithium transport through the grain boundaries to the active site. The dopant also acts to increase the overall capacity of the battery incorporating the electrode material. It is also believed that the presence of dopant increases the efficiency of forming the material.
[0011]
The dopant can include a metal alone, a metal oxide alone, a metal nitride alone, a metal oxynitride alone, or a combination of the foregoing. Similarly, the dopant can comprise a mixture of various metals and compounds, and in such cases, if the dopant comprises a metal and a metal compound, the compound can be of the same metal used in the dopant. It need not be a compound; however, in many preferred formulations it is understood that the metal forming the compound is the same as the companion metal of the dopant metal.
[0012]
It has been found that certain advantages are achieved when the materials of the present invention are present in a separate phase and the presence of a dopant facilitates the formation of such a separate phase. Specifically, zirconia (ZrO ₂ ) or zirconium suboxide will not be fully incorporated into the lattice of the non-oxidized ceramic of the present invention and will form an oxide nanodispersed phase throughout the material. I know that. Furthermore, the presence of this oxide will in many cases also cause the host ceramic material to be present as a nanophase dispersion as well. In this context of the present content, the nanodispersed phase is to be understood as a phase composed of regions having an average diameter of less than 10,000 mm, most preferably less than 5,000 mm. The presence of the nanodispersed phase is believed to reduce the activation energy required for lithium diffusion, thereby enhancing the performance characteristics of the material.
[0013]
The materials of the present invention have many uses, such as electrode materials, catalysts, and applications as a result of their stability, good electrical conductivity and new electrical properties. As mentioned above, one particular important application is the cathode material for rechargeable lithium batteries. The material of the present invention can be incorporated into the electrode by techniques well known in the art. As such, the material is typically disposed on a conductive support such as a metal foil, mesh or the like. The materials of the present invention can be deposited directly as thin films or formed layers, which are often utilized in powder form and preferably contain binders and the like. In certain particularly preferred embodiments, this material is ground to a fine powder, typically having a particle size of less than 25 μm. This powder is mixed with an inert binder such as 5-25% carbon (preferably acetylene carbon) and fluoropolymer particles.
[0014]
One particularly preferred material of the present invention comprises lithium, most preferably a mixture of zirconium and zirconium oxide, lithium vanadium nitride, optionally doped with zirconium nitride or oxynitride. In this type of material, zirconium has been found to be incorporated into the vanadium nitride lattice and cause lattice expansion or distortion to facilitate lithium transport therethrough. In addition, zirconium is a p-type dopant material and enhances the electrical conductivity of the vanadium nitride matrix. Zirconium is generally miscible with vanadium nitride at concentrations up to about 6 atomic percent. Zirconium oxide tends to form an oxide network with dispersed atoms that results in nano-dispersed oxide regions. These produce lattice discontinuities and perturbations that provide a lithium ion transport pathway. As a result, lithium diffusion through this material is very good, and batteries incorporating this material have both high capacity and fast charge / discharge. In addition, the lattice structure of these materials is very stable through repeated penetration and removal of lithium, so that batteries incorporating such materials have a long cycle life. The β value of the dopant species in certain preferred embodiments is approximately 0.06.
[0015]
Electrolyte degradation is a problem encountered when batteries incorporating prior art oxide-based cathodes are operated at high temperatures or high current conditions that cause heating of the electrolyte. The nitrogen-containing material of the present invention is much less prone to such electrolyte degradation and thus extends battery life. This effect is observed even when the electrode material contains a large amount of oxide-based ceramic, provided that the nitrogen-based ceramic covers most of the outer surface of the oxide particles. Thus, the material of the present invention can be used as a protective coating on prior art electrode surfaces, particularly in high temperature and / or high current applications.
[0016]
The materials of the present invention can be processed by conventional methods well known in the art used to process ceramics, particularly non-oxide ceramic materials. In one such group of technologies, an oxide-based precursor is first processed and subsequently treated with a suitable chemical reagent, for example, by high temperature reaction of the oxide under a stream of nitrogen-containing gas, nitrides or other non-reactive materials. Converted to oxide ceramic material. This type of technology is disclosed in US Pat. No. 5,680,292, the contents of which are incorporated herein by reference. One particularly preferred technique is the sol-gel process in which metal alkoxides are reacted in solution to form a metal oxide material gel. The gel is then dried to form a solid material that is then reacted in a nitriding atmosphere to produce the material of the present invention. In such reaction pathways, various components can be converted from oxides to nitrides at different rates, which is beneficial for the practice of the present invention. For example, in the preparation of the zirconium-doped lithium vanadium nitride material described above, the oxide of the metal is first formed and subsequently nitrided. The nitriding treatment of vanadium oxide is performed at about 600 ° C., while the nitriding treatment of zirconium oxide is performed at about 1600 ° C. This conversion process can therefore be carried out at intermediate temperatures, in which a significant part of the zirconium will still be present as oxides. This will form the aforementioned nanophase dispersed mixed dopant. Similar results will be achieved with other transition metals, particularly scandium, yttrium, hafnium and lanthanum. Sol-gel processing methods have been demonstrated in a number of previous references, for example, disclosed in US Pat. No. 5,837,630, which is incorporated herein by reference.
[0017]
The general principle of the present invention, the properties of the material, will be explained with reference to a specific group of materials consisting of lithium, vanadium, zirconium and even their oxides. It should be understood that these series of examples illustrate the invention but do not limit its practice. Other compositions are encompassed by the present invention, and their composition, use, characteristics and synthesis will be apparent from the present specification.
[0018]
One method of processing the material of the present invention is disclosed herein, but it should be understood that this may be implemented in other ways. In this synthesis, a zirconium / zirconia doped lithium vanadium nitride material was prepared as follows: 2.44 g of vanadium triisopropoxide was placed in a 100 ml beaker. A solution of 0.31 g of zirconium tetra-n-propoxide, 0.11 ml of ethanol and 0.06 g of acetylacetone was added to the vanadium alkoxide by the dropping method. This produced a clear yellow solution. To this yellow solution, 1 ml of a solution of 0.475 g of lithium methoxide using 5 ml of methanol as a solvent was added. This gave an orange product and the solution began to become slightly turbid after 1 minute. The remaining lithium methoxide solution was added, resulting in a cloudy orange solution with a very fine white precipitate. Two drops of 0.67 ml of water in 1.00 ml of ethanol were added resulting in a white precipitate; then 0.6 ml of water / ethanol solution was added and a large amount of white precipitate was formed which gradually dissolved. When the remaining alcohol solution (approximately 1.1 ml) was added to this solution, a white gel-like precipitate was formed and no free liquid was observed in the beaker. The gel was evaporated to dryness under a stream of nitrogen to obtain a porous yellow powder containing a mixture of lithium, vanadium and zirconium oxide.
[0019]
In the second step of the process, the oxide material was at least partially converted to a nitride material by treatment under high temperature ammonia atmosphere. Specifically, this material was placed in a reaction boat in an annular furnace through which 200 ml / min (200 cc / min) of ammonia atmosphere was flowed. The temperature of this material rose from room temperature to 300 ° C. over approximately 1 hour, and was then brought to 600 ° C. over approximately 3 hours. The temperature was maintained at 600 ° C. for 2 hours, after which the reaction was stopped by cooling to approximately 70 ° C. over 1½ hours, but the ammonia flow was maintained in all these steps. The ring is then flushed with argon at 100 ml / min (100 cc / min) until cooled to approximately 50 ° C., at which point the argon atmosphere is flushed with 1% oxygen in helium at a flow rate of 50 ml / min (50 cc / min). Replaced with atmosphere. This atmosphere was utilized for passivating the material surface and was typically applied for approximately 20 minutes. The resulting material comprised the doped ceramic of the present invention.
[0020]
In this process, ammonia atmosphere is used to convert various oxidizing materials to their corresponding nitrides; however, some oxides may still remain in the material, particularly in the center of the particles. It should be understood that the dopant zirconium is present as a free metal and / or oxide. It will be appreciated that other materials can be processed by changing the amount and / or type of reactants used. Furthermore, this conversion process can be carried out using reagents other than ammonia.
[0021]
A series of materials with a general composition of LiVZrON was prepared by the sol-gel method described above. The sample was subjected to powder x-ray diffraction analysis. This x-ray pattern was consistent with a material composed of a VN phase together with a separate phase based on ZrO ₂ . The data further shows that certain Zr is doped into the VN structure as part of ZrO ₂ . This x-ray diffraction data further suggests that in some cases, some of the lithium vanadium oxide material produced by the sol-gel process remains unconverted after nitrogen treatment. . These unconverted oxide materials, when present, are believed to form a core surrounded by the converted nitride material. Thus, it should be understood that the materials of the present invention may include a portion of the oxide material that in some cases is separated from the oxide dopant.
[0022]
The x-ray diffraction data also shows a peak that is indicative of the shift in diffraction of some VN matrices, and this peak is consistent with the presence of the extended portion of the VN lattice caused by the lattice extension by Zr. Yes. All x-ray data show that a portion of the dopant material, possibly metal, penetrates into and extends the transition metal nitride matrix, while the remaining dopant portion, possibly metal oxide, Consistent with materials that are configured to be utilized to form additional nanodispersed phases. The unconverted oxide material can also form another nanodispersed phase that is distinct from or incorporated into other phases.
[0023]
These materials were subjected to scanning electron microscopy using a JEOL T300 scanning electron microscope. No separate phase was observed in this material. Since the resolution limit of this microscope under the operating parameters used is approximately 500 nm; to the extent that multiple phases exist, this phase must be less than 500 nm. Therefore, those materials whose x-ray diffraction shown is multiphase must be dispersed in the nanophase.
[0024]
The electrochemical properties of the material thus prepared were evaluated. These materials were processed into sample cathodes. In this cathodic processing protocol, the material was screened to an upper particle size of 25 μm (500 mesh), 5% by weight of acetylene black was added, and 10% by weight of fluoropolymer (Teflon) was added. These materials were compressed on an aluminum current collector, incorporated into a Swagelok battery cell, and examined with an Arbin 8-channel automatic battery tester. The cell was cycled between 1.5 V and 4.0 V at 25 ° C. using 1: 1 PC: EC + 1 M LiPF ₆ electrolyte and lithium metal as the anode. Cycle voltammetry was performed immediately after battery processing (before constant current cycle) using a two-electrode arrangement and a scan rate of 0.2 mV / sec.
[0025]
After the first evaluation, the effect of the dopant concentration was evaluated. A series of samples were prepared such that the dopant level indicated by the subscript β in the above equation was varied. Samples were prepared with β values of 0, 0.06, 0.09, 0.18, 0.24 and 0.42. Constant current cycling was performed at different rates up to 12 hours, and for all rates, 0.06β material was found to show the maximum charge capacity when measured in mA · h / g. In general, the undoped material (β = 0) performed worse than the doped material, except that the β = 0.42 material was significantly inferior to the undoped material.
[0026]
Cycle voltammetry was performed on all of the aforementioned reference specimens, again β = 0.06 material was superior to any other, and β = 0.42 material was inferior to undoped β = 0 material.
[0027]
The structural stability of the material during the charge / discharge period was evaluated by Cu (Kα) x-ray powder diffraction method. It was observed that only a very slight change (less than 0.3 °) in the scattering angle 2θ occurred during the cycle, indicating that the maximum expansion of the basic unit cell of the material was only about 0.03 mm. . This indicates that the structural integrity of the host lattice is preserved during lithium penetration.
[0028]
The lithium diffusion coefficient of β = 0.06 material was found to be in the range of 1 × 10 ⁻⁹ to 10 × 10 ⁻⁹ cm ² / sec for ^uncycled materials. This diffusion coefficient was determined using a galvanostatic intermittent titration method in which a constant current pulse was applied to the material and subsequent open circuit relaxation was recorded. The sample was completely relaxed with respect to the open circuit potential between successive pulses, and the diffusion coefficient was calculated according to the following formula:
D = (4L ² / πτ) × (ΔE _δ / ΔE _τ ) ²
(Where D is the diffusion coefficient, L is the cathode thickness, τ is the pulse interval, ΔE _δ is the change in open circuit potential, and ΔE _τ is a constant between the start and end of the pulse. It is a transient potential difference and the resistance potential drop is negligible.)
[0029]
In another evaluation, the effect of particle size on cell performance was determined. One cell was prepared from a material having a random particle size, and another cell was prepared from a material that was sieved to include particles with an upper limit size of 25 μm. In general, the charge capacity of the screened material was greater than that of the random particle size material. It was also observed that the performance measured by the charge capacity increased slightly as the amount of acetylene carbon in the binder increased from 5 to 10% by weight. This effect was generally mild and higher at high charge / discharge rates.
[0030]
The β = 0.06 material was compared with prior art LiCoO ₂ , LiNiO ₂ and LiMn ₂ O ₄ materials that are dominant in the market as current lithium battery cathodes. The average capacity of the material of the present invention was 10%, 12% and 32%, respectively, over those of the prior art. When the material of the present invention was compared with the LiCoO ₂ electrode material, it was found that the material of the present invention would probably cause much less electrolyte degradation at high temperatures.
[0031]
The foregoing describes the general principles of the present invention, and in view of that, using another transition metal in place of vanadium and processing other materials using other dopant metals and oxides. It will be understood and understood that what can be done. Others of such systems have different optimum values for the dopant level, and such values can be readily determined without undue experimentation based on the content presented herein. it can. Accordingly, it should be understood that the foregoing discussion and examples are illustrative of specific embodiments of the invention and are not intended to limit its practice. The "claims" above, including all devices, define the scope of the invention.

Claims (11)

General formula Li _α V _1-β T _β N _x O _δ (where T is a dopant which is zirconium or titanium; x is greater than 0 and less than or equal to 1; δ is greater than 0 and less than or equal to 4 There; alpha is located largely in 3-x or less than 0;. and, beta will have a greater than 0 and less than 0.2), a portion of the dopant is doped to the nanophase vanadium nitride, the dopant A transition metal-based ceramic material in which the remaining portion of is present as a metal oxide of the dopant and at least a portion of the oxide is present as a nanodispersed phase .   The material of claim 1, wherein the dopant metal comprises zirconium.   The material of claim 2, wherein a portion of the zirconium is present in the material as a compound selected from the group consisting of zirconium oxide, zirconium nitride, and combinations thereof.   The material of claim 1, wherein the dopant metal is zirconium and β is 0.06. The battery cathode is of the general formula Li _α V _1-β T _β N _x O _δ (where T is a dopant which is zirconium or titanium; x is greater than 0 and less than or equal to 1; δ is 0 greater is 4 or less; alpha is located largely in 3-x or less than 0;. and, beta will have a greater than 0 and less than 0.2), a portion of the dopant, the nanophase vanadium nitride Re-doped, wherein the remaining portion of the dopant is present as a metal oxide of the dopant and at least a portion of the oxide is comprised of a transition metal-based ceramic material present as a nanodispersed phase. Rechargeable lithium battery. The battery of claim 5 , wherein the cathode comprises a compact of particles of the ceramic material, the particles having a size not exceeding 25 μm. The battery according to claim 5 , wherein the cathode includes a compressed body formed of particles of the ceramic material together with carbon particles. The battery according to claim 7 , wherein the carbon includes acetylene carbon. The battery of claim 7 , further comprising a fluoropolymer as a binder.   The material according to claim 1, wherein δ is 2 or less.   An electrode comprising the material of claim 1.